I thought it would be nice to put together an overview of the techniques you can use to fix or enhance brightness or contrast in an image. Feel free to suggest your tricks!

1) Colors > Brightness/Contrast. Very basic. Many times this is too all or nothing though.

2) Colors > Levels. One step more advanced. Often times is sufficient to do most basic corrections. Usually you'll want to drag the end arrows in to remove dead-space on the histogram and then play with the middle triangle until you get a nice look. Cropping some of the histogram is usually ok...but not too much.

3) Colors > Auto > Normalize. This will stretch the histogram to destroy deadspace to provide nice white balance and won't destroy any color data. This is usually very safe and a precursor to other operations...but often times it is too passive.

4) Colors > Auto > White Balance. Unlike normalize, this will chop weak data to provide a nice white balance. Sometimes this works well, but sometimes it doesn't. It also corrects color as well. I believe this is the same operation as Colors > Levels > Auto. Generally speaking I prefer manual levels over the auto functions unless I need to do batch operations (one of the major gimp batch plugins supports auto levels).

3) Colors > Curves. One step more advanced than levels. Through repeated sampling and adjusting you can more precisely control the shadows and highlights. Downside like levels is that this can be too all or nothing.

4) Using selections. This is a powerfull technique for brightness/contrast fixes. If I take a picture into the sun, no doubt I'll have exagerated highlights and shadow. Using curves/levels will not let me say restore detail of say a dark raft in the shadows without ruining the surrounding water and sky. Selecting the raft first, and then doing basic color operations on just the selection (like white-balance with levels) can produce VERY nice results. Downside is that it can take a while to make nice selections.

5) Filtering color corrections using HSV layers. If you duplicate your base layer and put it at the top with layer mode set to either Hue, Saturation or Balance...then those respective attributes will be protected. This is often handy when doing brightness tweaks, and you want to protect Saturation or Hue. Dupe layers for these...and then set one mode to Saturation...one to Hue. You can then see if that helps. Using protection layers is applicable for nearly all the techniques discussed here.

6) Overlay mode + duplicate layer. Overlay mode for layers provides more contrast. If you don't have blownout highlights or hidden shadows, this can be a very nice way to provide 'pop' to your picture. But if you do have shadow/highlight issues, this can exagerate those problems. Soft-light is a less mild version of overlay. Overlay mode can negatively affect colors though. It can be benificial to put a another layer ontop set to 'color' so overlay only affects brightness.

7) Contrast Mask. The idea of a contrast mask is to reduce blownout areas while similantously brighten under-exposed shadows. This is a very effective technique for a lot of amature outdoor shots. Nice tutorial at: http://www.gimp.org/tutorials/ContrastMask/ In a nutshell, you duplicate a layer, place it on top, desaturate it, invert it, blur it (like like radius 10px), then set the top layer to overlay. Normally overlay increases contrast, but because the brightness is the opposite, it decreases it. If your picture doesn't have shadow/highlight issues, this can unfortinitely make a picture look less 3d and more flat (you are reducing contrast after all).

7a) While most contrast mask plugins didn't recreate the same look as the manual technique describe above, the closest and best IMO is FX foundary > Photo > Enhancement > Contrast Overlay (I like blur = 10 and opacity = 100%)

8) Luminosity mask. These are apparently legendary and I'm still trying to figure them out. The gist is that you sub-divide a picture into say 2-5 selections based on lightness. You then perform curve corrections separately on each selection or mask. Great way to protect say a sky while adding that detail back into the shadows. Core concepts here: http://goodlight.us/index.html This is a very popular technique with nature photography.

8a) Obtaining tonal regions using thresholds and layer masks. In theory this should work, but I get banding issues.

8b) Generate the luminosity masks using this tool: http://registry.gimp.org/node/25479 Not bad, but seems to lack the ability to make more fine masks.

8c) G'mic > Enhancement > Mask Creator . Seems to be powerful and there is a nice example at http://www.flickr.com/groups/gimpusers/ ... 890498_281 ...but I just can't get a handle on this.

8d) Manually using desaturation/invert/difference layers. Nice but only gives you three tonal selections. http://gimpforums.com/thread-luminosity ... mp#pid5807

9) Tone mapping. Tone mapping basically reduces global contrast, while increasing local contrast. This is often times perfect! Example, I take a picture in the woods...normally the sky is overexposed and the trees underexposed. This balances the sky and gives it a nicer darker blue. Then it brightens the forest flow, but keeps local contrast so say the bark on the trees really stands out. Great trick!

9a) Think gimp comes with a tone mapping filter, but wouldn't recommend it

9b) Advanced Tone Mapping Plugin: http://registry.gimp.org/node/5980 Very nice. Similar effect to a contrast mask...but perhaps doesn't recover as much detail from the shadows and doesn't add as much in saturation.

9c) This is a different variant that is manual but allows for some nice tweaking. I liked the results. http://www.youtube.com/watch?v=FMlKVDjJFfY It provides a little bit more saturation then the other tone mapping techniques.

9d) G'mic has a nice one. G'mic > Colors > Tone Mapping (lots of options)

10) Shadow recovery. Filters > Eg > Shadow Recovery. Not bad results for taking on shadows.

11) Use mid-tone selections for levels/curves to protect the extreme black/white ends of your picture. Often times you just want to bring out the detail in the image without messing up the extreme dark and light portions of the picture. Duplicate your picture and use an auto-normalize, to apply a quick but safe white-balance. Right click the top layer and create a new layer mask from a grey scale version of that layer. Edit the layer mask with curves to give it a pulse shape (0->255->0). This will now only select the midtones of your top layer. Now select the top layer itself (not the mask) and play with levels/curves. You can now increase the contrast of the midtones safely.

Can you try and summarize the main idea of what your saying?

The key to making any picture look nice, give it more pop or just look more professional often comes down to brightness and contrast settings.

There are many different techniques in Gimp to adjust brightness & contrast, many with their own pros and cons.

The above list are various methods you can try to enhance your pictures to make them look the best.

White overlays and black layers set to multiply then mask out areas that shouldn't be affected works in most cases and is as subtle or drastic as pulling down a layers opacity. Any of those listed will work though and they work to some degree or another. It really always depends on what you want to achieve and how much time you want to/can spend fiddling with it.

Those are some great tips and i wonder if i should move this post to the Gimp Tips and Tricks topic?

Rod, I think it is an excellent post with lots of great alternatives. I would put it into Tips and tricks. It would be more useful there than in Tutorials.

Thanks for your clarification, great post.

@fb
following a tutorial read here, some time ago, I also use:
1-duplicate
2-border average (I use 3, 4 as parameters) [under: Colours/Info]
3-colour to alpha [under: Colours]
4-overlay, normally using 50%opacity
cheers

Something I just remembered that works and I've discovered during my digital makeup phase is the color to alpha. You can strip the midtones right out of that if you desaturate, then color to alpha at 50% grey first, and only then remove either the black or the white after. This will give you a nice deep contour and really glossy highlight. It's pretty intense but again, layer opacity can take the edge off of it like anything else.

Thank you for the totally excellent information. I appreciate that you took a considerably long time to do it and make it clear and understandable. This is really great!

ty, nice resource!

Not to be too nitpicky, but "Soft Light" and "Overlay" currently do the exact same thing...

For contrast, I really like MM Filter's Sigmoidal Contrast.

http://www.mm-log.com/blog/2010-05-09/s ... trast-gimp

Lyle it seems that link no longer works.

Rod, me too would like to experiment it
it seems to be something old 5 years...

These links to the files no longer work either.
You can find mm filters here though.
http://sourceforge.net/projects/mmfilters/files/LabCurves/

OK, thanks
downloaded
are there some instructions on the use?

edit: found some
------------------------------------------------
Process Steps:

1) Open the image
2) Filters > MM-Filters > Dynamic range compression
- Parameters: Alpha 1.0, Beta 0.5 (default values)
3) Filters > G'MIC > Colors > Lab Mixer
- Parameters: A-color gain 1.35, B-color gain 1.43
4) Filters > MM-Filters > Sigmoidal Contrast
- Parameters: Strength 5, Threshold 0.5

-------------------------------------------------------------
but under Filter -> MM-Filters I found only Lab curves
neither Dynamic range compression
nor Sigmoidal Contrast

edit2: it seems that SourceForge only provides the download for Lab curves
- Dynamic Range compression is another piece of MM-Filters?
- Sigmoidal Contrast is another piece of MM-Filters?

from where to download them if original links are broken?

Labcurves does all of it now i believe.

I can't get either to run correctly. The variables in both python files seem to be incorrect for pdb.file_tiff_save2.
It runs but nothing happens other than a new layer is created.

You can however do this manually (i believe) with Colors>decompose and decompose to LAB.
Now use curves on each layer and recompose the image.
Top LAB recomposed layer is in Lighten Only mode.
Original - Thanks to Molly.


Results after vibrancy touchup.


Too much blue i know but you see where you can go with this adjustment algorithm.


LAB adjusted layer at 50% opacity in grain merge mode-

thanks, Rod
now my problem is: learn using LAB!
is there a simple tut for that? I'm really a novice, not even understanding which values operate on the A-B channels (the simplest to understand for me is the L...)

have to go back to school...